Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistive, e.g., resistance variable, memory, among others. Types of resistive memory include programmable conductor memory, resistive random access memory (RRAM), and phase change random access memory (PCRAM), among others.
Memory devices such as resistive memory devices may be utilized as non-volatile memory for a wide range of electronic applications in need of high memory densities, high reliability, and low power consumption. Non-volatile memory may be used in, for example, personal computers, portable memory sticks, solid state drives (SSDs), digital cameras, cellular telephones, portable music players such as MP3 players, movie players, and other electronic devices.
Memory devices such as resistive memory devices may include a number of memory cells, e.g., resistive memory cells, arranged in an array. For example, an access device, such as a diode, a field effect transistor (FET), or bipolar junction transistor (BJT), of the memory cells may be coupled to an access line, e.g., word line, forming a “row” of the array. The memory cell material, e.g., memory element, of each memory cell may be coupled to a data line, e.g., bit line, in a “column” of the array. In this manner, the access device of a memory cell may be accessed through a row decoder activating a row of memory cells by selecting the word line coupled to their gates. The programmed state of a particular memory cell in a row of selected memory cells may be determined, e.g., sensed, by causing different currents to flow in the memory elements depending on the resistance associated with a programmed state for the particular memory cell.
Memory cells such as resistive memory cells may be programmed, e.g., written, to a desired state. That is, one of a number of programmed states, e.g., resistance levels, can be set for a memory cell. For example, a single level cell (SLC) can represent one of two logic states, e.g., 1 or 0. Memory cells can also be programmed to one of more than two programmed states, such as to represent more than two binary digits, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, or 1110. Such cells may be referred to as multi state memory cells, multi-digit cells, or multilevel cells (MLCs).
Resistive memory cells such as RRAM cells and PCRAM cells may store data by varying the resistance level of a resistive memory cell material, e.g., resistive memory element. For example, data may be programmed to a selected RRAM cell by applying sources of energy, such as positive or negative electrical pulses, e.g., positive or negative voltage or current pulses, to a particular RRAM cell material for a predetermined duration. RRAM cells may be programmed to a number of resistance levels by application of voltages or currents of various magnitudes, polarities, and durations.
The resistive memory cell material of a PCRAM cell may be a phase change material, such as Germanium-Antimony-Telluride (GST). The phase change material may exist in an amorphous, higher resistance state, or a crystalline, lower resistance state. The resistance state of the PCRAM cell may be altered by applying sources of energy to the cell, such as current pulses or pulses of light, among other sources of energy. For example, the resistance state of the PCRAM cell may be altered by heating the cell with a programming current. This may result in the PCRAM cell being programmed to a particular resistance state, which may correspond to a data state. In a binary system, for example, the amorphous, higher resistance state may correspond to a data state of 1, and the crystalline, lower resistance state may correspond to a data state of 0. However, the choice of these corresponding data states may be reversed, that is, in other binary systems, the amorphous, higher resistance state may correspond to a data state of 0, and the crystalline, lower resistance state may correspond to a data state of 1.
In previous approaches, resistive memory cells, such as RRAM cells and PCRAM cells, in a resistive memory device may be horizontally oriented, e.g., the electrodes of the cells may be located adjacent the sides of the cells. Additionally, in previous approaches, resistive memory cells in resistive memory devices may be vertically stacked, e.g., a first cell may be stacked on top of a second cell. However, in previous approaches, horizontally oriented resistive memory cells may not be also be vertically stacked, and vice versa. That is, a memory device processed in accordance with previous approaches may have only horizontally oriented memory cells or only vertically stacked memory cells, but may not have memory cells that are both horizontally oriented and vertically stacked. Memory devices having only horizontally oriented memory cells or only vertically stacked memory cells may have a large size, e.g., area, and/or a low density.